The overall goals of the proposed research are to gain a better understanding of how the nervous system coordinates the trunk to maintain equilibrium, and to determine if subjects with idiopathic low back pain have abnormal coordination. A movable platform will be used to translate the support surface and perturb equilibrium. The resulting response to maintain balance will be examined by extensively measuring trunk kinematics, surface reaction forces, and the activity of many trunk muscles. The first specific aim is to examine normal subjects: (A) To determine the overall kinematic and muscular strategies for stabilizing the trunk during sitting and standing in response to perturbations in different directions. This will address is there are different strategies and whether they are similar for sitting and standing; (b) To determine the degree to which the nervous system controls trunk muscles together or in fixed patterns. This will be determined by measuring spatial and temporal correlations of trunk muscles, and will address how the nervous system controls the large number of back muscles. The second specific aim is to determine if selected groups of patients with idiopathic low back pain have abnormal coordination of the trunk. To avoid confounding different pathologies, this study will only examine patients which fall into two homogeneous classes based on clinical measures. Using the same protocols as normals, the proposed study will examine whether either group of low back pain patients has abnormal motor control, and if abnormalities are different between the two groups. Through measuring kinematics, forces and muscular activity, this study will increase our basic knowledge of how the trunk is controlled and determine if subjects with low back pain have altered motor control. A detailed characterization of the abnormalities of low back pain subjects will provide a basis to examine causal relationships between motor control and back pain and to develop therapeutic interventions.